Digestive System

By the end of this class you should understand:

• The processes of physical and chemical digestion

• The route of food, nutrients and waste through the body

• The major nutrients the body requires for life• The major risks associated with excess and

deficiencies of food

Digestive Organs

• The digestive system is divided into two types of organs:– The alimentary organs that

make up the alimentary canal– Accessory organs that do not

make up the alimentary canal

• The alimentary canal is the tube running through your body

Alimentary Organs

• Mouth• Pharynx (throat)• Esophagus• Stomach• Small intestine• Large intestine• Anus

Movement

• Organs are moved from one to the next through a process called peristalsis (the pushing of food through the alimentary canal by smooth muscle)– In the esophagus, swallowing is

initiated by skeletal muscle but completed by smooth muscle and qualifies as peristalsis

– The stomach, small and large intestine all have peristalsis but it is much slower

Digestive Function• The digestive system has

one goal only: to put nutrients into the bloodstream so that all our cells can access those nutrients– Many excess nutrients

can be stored• The digestive system has

many “preparation steps” to prepare food for this process

The Most Vital Organ• Only one organ performs

almost all the absorption of nutrients (excepting water): the small intestine– So called because it has a

small diameter, though it is several meters long

• If we drank liquids containing only single molecules of nutrients, we would need no other digestive organ!– This is what they inject into

your blood when you’re in a coma

Two Types of Digestion• Digestion is the process of breaking

down nutrients so they can be absorbed– Undigested nutrients cannot be

absorbed and pass through the small intestine to the large intestine

– For more information, ask someone who is lactose-intolerant about drinking milk

• Mechanical (physical) digestion is the chewing and churning of food

• Chemical digestion is the breaking of chemical bonds in food

Mechanical Digestion

Chemical Digestion• The mouth and stomach also

initiate the process of chemical digestion– The mouth has salivary glands that

produce saliva, which begins chemical digestion

– The stomach has pepsin, an enzyme that denatures and breaks down protein

• Note chemical digestion of fats does not begin until the small intestine

The Stomach• The stomach produces

gastric juice, which contains pepsin as well as other chemicals

• This includes hydrochloric acid, which makes the stomach environment very acid

• The acid itself is not for digesting the food– Kills bacteria– Enables pepsin to work

Stomach Ulcers

• Stomach ulcers were once thought to be caused by stress but are now known to be a bacterial infection (but stress does block the immune system)

• The scientist who discovered the bacterium that causes ulcers was eventually awarded the Nobel Prize– No one had believed him initially so

he drank a flask of the bacterium to gain ulcers

Gastric Juice

• Gastric juice is the secretion of the stomach• Gastric juice is very acidic and can damage

tissues other than the stomach lining– Repeated vomiting can cause gastric juice to

erode the enamel of the teeth– If gastric juice is produced in excess it can

overflow into the esophagus, causing “heartburn”• Once gastric juice mixes with and digests food

it is called chyme

Processing of Chyme• Chyme is the partially

digested food of the stomach mixed with acid

• Chyme is delivered into the small intestine very slowly– Excess acid could damage the

small intestine– Helps the intestine absorb all

the nutrients• The small intestine has many

digestive enzymes of its own that complete digestion

Accessory Organs• The small intestine has enzymes on

its absorptive cells but also receives digestive juices from two accessory organs

• The pancreas produces a massive battery of digestive enzymes to finish all digestion of food– Most prominent enzyme is trypsin, a

protease that breaks down protein into amino acids

• The liver produces bile, which is stored in the gallbladder until chyme enters the small intestine

Bile• Bile does not contain

digestive enzymes and does not directly perform chemical digestion

• Bile is instead an emulsifying agent, much like a detergent– To emulsify is to allow oil and

water to mix• Bile is therefore vital for

digesting and absorbing fats from food

Small Intestine Absorption• Since nutrients can only be

absorbed through cell membrane, the small intestine epithelial tissue has adaptations for increasing surface area– The tissue is arranged into fingerlike

villi (singular villus) which create more surface area

– The individual cells have a brush border, or microvilli (singular microvillus)

• These cells expend ATP to actively transport nutrients into the blood

Liver Function• Blood flows directly from the

small intestine to the liver– This is called hepatic portal

circulation– Protects the rest of the body

from whatever you just ate• The liver has metabolic enzymes

to process many nutrients after they are absorbed– Production of fat and glycogen– Storage of vitamins– Breakdown of toxins such as

alcohol

Large Intestine• Once all nutrients are absorbed into

the small intestine, the large intestine (which is mostly composed of a tube called the colon) carries remaining indigestible materials to the anus

• The large intestine is filled with colonies of bacteria that feast on our leftovers (often producing gas as a waste product)

• The large intestine is capable of absorbing water and vitamins, including those produced by bacteria

Vital Nutrients• The small intestine absorbs all

the nutrients required for life• Some nutrients are required for

energy and for building blocks for more cells and cell parts– Carbohydrates– Proteins– Lipids

• Other nutrients are required for homeostasis but not energy– Water– Vitamins– Minerals

Carbohydrates• A single carbohydrate molecule is

called a simple carbohydrate (such as glucose)– Starches are made of many bound

together and are called complex carbohydrates

– Some are more complex than others, and the more complex the branching, the longer the digestion takes

• “White” starches such as white rice and pasta contain simpler chains– Complex carbs, since they break down

more slowly, are less likely to cause blood sugar to spike

Proteins• Proteins must be broken down to

the individual amino acids• The human body can synthesize

many of the amino acids from other amino acids (so they are nonessential), but there are certain ones that cannot be synthesized– These are called essential amino

acids• If even one of the essential amino

acids is missing, the diet is incomplete and no new protein can be made

Lipids• There are many classes of lipids– Some vitamins and cholesterol are

both lipids, but cannot be burned for energy

• The energy-storing lipids are fatty acids, and these fats can be saturated or unsaturated– Saturated fats have no double bonds

and so pack together more tightly than unsaturated fats

– Treating unsaturated fats with hydrogenation can create trans fats, which are very difficult to metabolize and therefore very unhealthy

Vitamins and Minerals• Vitamins are organic

compounds that serve key purposes in cell mechanisms but only in small amounts– Example: vitamin A used to

make retinal• Minerals are inorganic

elements and compounds, such as sodium, potassium, calcium, iron, and iodine– Also necessary for life

Hopefully that wasn’t too much to digest…

• Enjoy your Thanksgiving weekend!• See you next week for the last week of new

lessons!– Tuesday: Urinary system– Thursday: Reproductive system